The present invention relates to an automatic power transmission for an automotive vehicle. The present invention particularly relates to lock-up control apparatus and method for a torque converter of the vehicular automatic power transmission associated with an internal combustion engine which is driven in a plurality of driving modes. The associated engine is such a lean burn engine or internal-cylinder direct injection gasoline engine that the different driving modes make different engine output torque values under exactly the same engine load applications.
A torque converter is installed in an automatic power transmission in order to provide a driving torque augmentation function and a driving torque variation absorption (compensation) function for a drive train in the automatic power transmission.
Since, however, a slip occurs between input and output elements (pump impeller and turbine liner) of the torque converter, a power transmission efficiency becomes unavoidably worsened.
Hence, many of the torque converters, in these days, adopt a lock-up type such that, when the vehicle is transferred into such a driving condition that neither the torque augmentation function nor torque variation absorption function is required, a converter state in which no slip limitation occurs between the input and output elements can be switched to a lock-up state in which a mechanical (direct) coupling between the input and output elements occurs.
It is a general practice that, in a lock-up control method for such a lock-up type torque converter as described above, a controller determines whether the torque converter should fall in the lock-up state or not according to the engine load state and a vehicle speed, including the case where the engine located at a prior stage of the torque converter is such the lean burn engine or internal-cylinder direct injection gasoline engine as is driven in the plurality of the driving modes such that the different driving modes make the different engine torque values under the same engine load conditions.
A Japanese Patent Application First Publication (unexamined) No. Showa 59-150942 published on Aug. 29, 1984 exemplifies a previously proposed control apparatus for an air-fuel mixture ratio of a lean burn engine associated with the lock-up type torque converter.
In the above-identified Japanese Patent Application First Publication, the air-fuel mixture ratio of the engine becomes lean, i.e., a lean burn state when the lock-up state of the torque converter occurs.
However, in the lock-up control technique for the torque converter associated with the lean burn engine described in the above-identified Japanese Patent Application First Publication, a lock-up region to render the torque converter in the lock-up state which is determined according to the engine load and the vehicle speed is left unchanged in both two driving modes such that in a lean burn state the engine output torque value becomes small and in a lean burn inhibit state wherein the air-fuel mixture ratio is maintained at a stoichiometric air-fuel mixture ratio the engine output torque value becomes large, under the same engine load conditions.
That is to say, since the lock-up region for the torque converter is the same in the case of the lean burn state of the engine and in the case of the lean burn inhibit state of the engine, the torque converter is locked up in such a low fuel consumption mode that the engine output torque is relatively low.
On the other hand, since it is an usual practice in the lean burn engine that when an engine coolant temperature is relatively low or relatively high, the lean burn state is inhibited. During such a lean burn inhibit state as described above, the engine output torque value is large as compared with the case of the lean burn state.
Hence, since an input torque imposed on the torque converter is large during the lean burn inhibit state of the engine, a pump drainage of a lock-up clutch piston used to directly couple between the input and output elements of the torque converter becomes lack so that a clutching force thereof becomes deficient, thus a, so-called, judder being developed.